1. Field of the Invention
This invention is concerned with switching type power supplies and is directed to converter circuits having transformer isolation between the input and output circuits. It is concerned particularly with single-ended converters and is specifically applicable to forward type, single-ended converters which are self-oscillating.
2. Description of the Prior Art
Forward type or transformer coupled, single-ended converters are a well-known class of converter circuits. A discussion of its characteristics may be found in the text Transistor Inverters and Converters, by Thomas Roddam, D. Van Nostrand Co., Inc., Princeton, New Jersey (1963) First Edition. The forward type, single-ended converter is characterized by a source of DC voltage, a switching device coupling the DC voltage to the primary of a power transformer, and a rectifying diode coupling the secondary of the power transformer to a load to be energized. The rectifying diode is poled to conduct when the switching device in the primary conducts. Hence, current is applied to the load by transformer action when the switching device is conducting.
In contrast to the more familiar ringing choke type, single-ended converter, very little magnetic energy is stored in the power transformer. The forward type, single-ended converter has very good voltage regulation characteristics and is very economical in terms of circuit components needed since only one active switching device and its associated biasing circuitry is required.
In applications in which electromagnetic interference is particularly undesirable, the switching device of the forward type, single-ended converter must be operated with relatively long turn on and turn off times. At short turn on and turn off times the switching device generates significant electromagnetic interference in the secondary of the converter. Current conduction in the secondary is abruptly terminated by the rapid turn off of the switching device in the primary, and electromagnetic interference is generated in the secondary due to rapid back biasing of the output diode. The forward type, single-ended converter needs an output filter inductor since this circuit tends to generate more ripple in the output current than the corresponding ringing choke type converter.
It is therefore an object of the present invention to reduce the electromagnetic interference generated by fast rise times in the output current waveshapes in single-ended converters at high operational frequencies.
It is also an object of the invention to operate a forward type, single-ended converter without the necessity of a large filter inductor in the output circuit.